Past colon cancer tied to future cancer risks

NEW YORK (Reuters Health) - People who have had colon cancer are 15 percent more likely to be diagnosed with another cancer than those with no history of the disease, a new study suggests.

Using data from cancer registries from nine states, researchers found small intestine, lung, kidney, stomach, bladder and endometrial cancers were all more common among people with a history of colon cancer.

"The most common cancer among these individuals is another colorectal cancer, but there are potentially other cancer sites that we need to be aware of," said Amanda Phipps at the Fred Hutchinson Cancer Research Center in Seattle, who led the study.

According to the American Cancer Society, about one in 20 people will be diagnosed with colon or rectal cancer during their lifetime.

The new study included about 170,000 people diagnosed with those cancers between 1992 and 2009. During that span, 15,000 of them - or about nine percent - were found to have a second cancer.

Compared to the average American, people in the study with a history of colon cancer were twice as likely to develop a second colon cancer.

They also had four times as many cancers of the small intestine as would be expected in a typical population and were 42 percent more likely to be diagnosed with kidney cancer and 14 percent more likely to get lung cancer.

Those risks varied by where in the colon or rectum a person's first tumor had been found, according to findings published in the journal Cancer.

"We have this growing knowledge that colorectal cancer isn't just one disease," Phipps told Reuters Health.

"We saw the greatest increased risk among individuals whose first cancer was located in sort of the central portions of the colon."

Although it's not clear why that was the case, she said the finding could help researchers in the future target patients who are most likely to get a second cancer.

"The general message is very consistent now over several papers, that colorectal cancer patients have good outcomes, however even when cured they have an excess risk" of future cancers, said Dr. Jason Zell, who has also studied the topic at the University of California, Irvine.

He said there are a few possible explanations for the findings. First, treatment of colon cancer - whether with surgery, radiation or chemotherapy - could raise or lower the risk of cancer in nearby organs.

Another possibility is that more cancers are caught among people with a history of disease simply because they undergo more tests than the average patient.

Or, he told Reuters Health, "Maybe we've been underestimating the effects of common risk factors, like genetics... and also just diet and lifestyle."

Zell, who wasn't involved in the new research, said current studies are looking into whether second cancers can be prevented through exercise or healthier eating.

He said the findings shouldn't change treatment of colon cancer survivors, including how closely they are watched for other diseases. That's in part because there are no good screening tools for some of the cancers tied to colorectal cancer.

"For right now, I don't think it affects any screening recommendations directly," he said.